Multi-wavelength light source and discrete-wavelength-variable light source

ABSTRACT

In a light source for generating light containing multiple wavelengths substantially uniform in intensity, a wavelength demultiplexing element  10  (for example, waveguide-type wavelength selecting filter) demultiplexes input light into a plurality of wavelengths λ 1  through λ 32 . Optical amplifiers  14 - 1  through  14 - 32  amplify outputs of the element  10  and applies them to input ports of a wavelength multiplexing element  12 . The wavelength multiplexing element  12  wavelength-multiplexes their input. Output of the wavelength multiplexing element  12  is applied to a fiber coupler  16  which, in turn, applies one of its outputs to the wavelength demultiplexing element  10 . The optical amplifiers  14  have a gain larger by approximately 10 dB than the loss in the optical loop made of the element  10 , optical amplifier  14 , element  12  and fiber coupler  16 . The other output of the fiber coupler  16  is wavelength-multiplex light containing wavelengths λ 1  through λ 32.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a division of co-pending application, Ser.No. 08/932,222, filed Sep. 17, 1997.

FIELD OF THE INVENTION

This invention relates to a multi-wavelength light source and adiscrete-wavelength-variable light source, and more particularly, to amulti-wavelength light source for supplying one or more optical outputswith different wavelengths concurrently or selectively and adiscrete-wavelength-variable light source capable of selecting one of aplural of wavelengths, which are suitable for transmission or tests of awavelength division/multiplex transmission system.

BACKGROUND OF THE INVENTION

In wavelength division multiplex transmission systems, it is essentialto reliably obtain laser lights with a number of close wavelengths. Fortransmission tests or tests of optical components used in wavelengthdivision/multiplex transmission systems, there is the need for a laserlight source highly stable in wavelengths and outputs.

ITU has recommended 0.8 nm (100 GHz) as the wavelength interval inwavelength division multiplex transmission systems. While temperaturecoefficients of wavelength changes of semiconductor lasers areapproximately 0.1 nm/° C. That is, semiconductor lasers are verysensitive to temperature fluctuation. Therefore, it is difficult tomaintain wavelength intervals of 0.8 nm in a number of semiconductorlaser light sources over a long period. Moreover, in ordinary lasersources, injected current is used to stabilize optical outputs. Controlcurrent for stabilization of optical outputs causes changes intemperature, and it results in changes in wavelength. That is, controlof optical outputs affects wavelengths, and makes it difficult tostabilize wavelengths.

A prior proposal to cope with the problem is to connect an opticalfilter and an optically amplifying element in a ring to form amulti-wavelength light source for collectively supplying multiplewavelengths. FIG. 15 is a schematic block diagram showing a priorexample A Fabry-Perot optical filter 210, erbium-doped optical fiberamplifier 212 and optical fiber coupler 214 are connected to form aring.

FIG. 16 show characteristic diagrams of the prior example of FIG. 15.FIG. 16(1) shows transparent wavelength characteristics of theFabry-Perot optical filter 210, FIG. 16(2) shows amplifyingcharacteristics of the optical fiber amplifier 212, and FIG. 16(3) showsthe spectral waveform of output wavelength. The Fabry-Perot opticalfilter 210 is a kind of wavelength selecting optical filters havingwavelength transparent characteristics which permit specific wavelengthsin certain wavelength intervals called FSR (Free Spectral Range) to passthrough as shown in FIG. 16(1). Individual transparent wavelengths ofthe Fabry-Perot optical filter 210 are selected from the spontaneousemission light generated in the optical fiber amplifier 212. The outputspectral waveform coincides with that obtained by multiplying thetransparent wavelength characteristics of the optical filter 210 by theamplifying characteristics of the optical fiber amplifier 212.Theoretically, laser oscillation outputs are obtained in wavelengthswhere the gain of the optical fiber amplifier 212 surpasses the loss ofthe optical loop.

In the prior art example shown in FIG. 15, the output intensity is largenear the gain center wavelength within the amplifying range of theoptical fiber amplifier 212, where oscillation is most liable to occur,and largely decreases in peripheral portions, as shown in FIG. 16(3).That is, the prior art example cannot realize simultaneous oscillationin multiple wavelengths in substantially uniform output levels.

Moreover, wavelength interval in output light in the prior art exampleexclusively depends on transparent characteristics of the Fabry-Perotoptical filter 210. When the wavelength interval is 0.8 nm (100 GHz),the wavelength interval FSR of the transparent wavelengthcharacteristics of the Fabry-Perot optical filter 210 is less than theuniform extension width of the erbium-doped optical fiber amplifier 212.Therefore, even when a plurality of oscillation wavelengths are obtainednear the gain center wavelength of the erbium-doped optical fiberamplifier 212, mode competition occurs, and results in unstable outputintensities and wavelength fluctuations of respective wavelengths.

A Fabry-Perot semiconductor lasers is a multi-wavelength light source,other than the fiber ring light source. However, it involvesunacceptable fluctuations in oscillation wavelengths due to modecompetition or mode hopping, and fails to uniform intensities ofrespective oscillated wavelength components.

SUMMARY OF THE INVENTION

It is therefore an object of the invention to provide a multi-wavelengthlight source and a discrete-wavelength-variable light source capable ofsimultaneously or selectively outputting one or more wavelengths with auniform intensity.

Another object of the invention is to provide a multiwavelength lightsource capable of selecting one or more wavelengths among a plurality ofwavelengths.

Another object of the invention is to provide a multiwavelength lightsource and a discrete-wavelength-variable light source immune totemperature fluctuations.

The invention uses a wavelength demultiplex/amplify/multiplexing unitfor demultiplexing input light into a plurality of differentpredetermined wavelengths, optically amplifying individual wavelengths,and multiplexing the wavelengths, and connects its output to its inputto form an optical loop. Since individual wavelengths are opticallyamplified by the wavelength demultiplex/amplify/multiplexing unit, laseroscillation in a plurality of wavelengths with substantially the sameintensity is promised in the optical loop. Since the structure is simpleand the most elements are passive ones, it is highly stable againsttemperature fluctuations.

When using wavelength demultiplexing means for demultiplexing inputlight into a plurality of predetermined wavelengths in predeterminedwavelength intervals, the light containing multiple wavelengths insubstantially constant wavelength intervals can be obtained. Usable asthe wavelength demultiplexing means is a waveguide-type wavelengthselecting filter, for example.

When using the optical band pass filter means which is transparent onlyto light within a predetermined wavelength band, the light source canprevent that light beyond the desired wavelength band circulates in theoptical loop. This contributes not only to stabilization of laseroscillation but also to reliably preventing that the output containsundesirable wavelengths.

By using optical modulation means for intensity-modulating circulatinglight in the optical loop with a modulation signal having a frequency,which is an integer multiple of the circulation frequency (namely, c/nL)in the optical loop, the light source can conjoin multiple-wavelengthlight into pulsating light synchronous with the modulation signal.Location of the optical modulation means may be either posterior towavelength division or posterior to wavelength multiplexing. When it islocated after wavelength division, fine adjustment of individualwavelengths,is easier, but a plurality of optical modulating means forindividual wavelengths must be used. When it is located after wavelengthmultiplexing, optical modulating means may be only one, but adjustmentof individual wavelengths must be done in another portion. Polarizationadjusting means may be provided in the input side of the opticalmodulating means to previously adjust polarization so as to ensureoptimum operations of the optical modulating means. If, of course,necessary means is of a polarization holding type, polarizationadjusting means may be omitted to reduce elements.

When individual optically amplifying means are capable of selectivelysupplying or blocking outputs to the wavelength multiplexing means,multiplex output light containing one or more selected wavelengths canbe obtained. If each of the optically amplifying means comprises anoptical amplifier for amplifying corresponding one of optical outputsfrom the wavelength demultiplexing means and an optical switch means forfeeding or blocking the optical output of the optical amplifier,undesired noise light is prevented from entering into the wavelengthmultiplexing means while the optical switch means blocks the path.

In another aspect of the invention, an output of wavelengthdemultiplex/amplify/multiplexing means for demultiplexing input lightinto a plurality of predetermined wavelengths, then optically amplifyingthem individually, and thereafter multiplexing them is connected to theinput of the same wavelength demultiplex/amplify/multiplexing means viapolarization means, optical dividing means and depolarization means toform an optical loop. There is also provided modulation means formodulating divisional optical outputs from the optical dividing means inaccordance with a modulation signal.

With this arrangement, light components with multiple wavelengths whichare simultaneously oscillated in the optical loop can be modulatedcollectively by the modulation means.

Since the polarization means suppresses fluctuations in plane ofpolarization, fluctuations in the ring cavity mode are less likely tooccur, and simultaneous oscillation in multiple wavelengths isstabilized. Since the polarization adjusting means in an appropriatelocation selects and maintains an appropriate plane of polarization foreach element, behaviors of individual elements are stabilized. Ifessential means are of a polarization holding type, polarization meansand polarization adjusting means may be omitted to reduce elements.

In another aspect of the invention, an output of wavelengthdemultiplex/amplify/multiplexing means for demultiplexing input lightinto a plurality of predetermined wavelengths, then optically amplifyingthem individually, and thereafter multiplexing them is connected to theinput of the same wavelength demultiplex/amplify/multiplexing means toform an optical loop and wavelength shifting means is provided in theoptical loop to slightly shift the wavelengths. As a result, laseroscillation is suppressed, and an ASE (Amplified Spontaneous emission)light source for multiple wavelengths can be realized.

Since the polarization adjusting means and depolarization means inappropriate locations select and maintain an appropriate plane ofpolarization for each element, behaviors of individual elements arestabilized. By taking out the light from the optical loop afterdepolarization, output light independent from or less dependent onpolarization can be obtained. If essential means are of a polarizationholding type, polarization adjusting means may be omitted to reduceelements.

In another aspect of the invention, an optical loop is formed such asdemultiplexing input light into a plurality of predeterminedwavelengths, then optically amplifying them individually, thereaftermultiplexing them and feedback to the input, and it is activated forsimultaneous oscillation in multiple wavelengths. Lights which arewavelength-demultiplexed and individually amplified are divided,individually modulated outside and thereafter wavelength-multiplexed. Asa result, multi-wavelength light containing individually modifiedwavelengths can be obtained.

In another version of the invention, an output ofselective-demultiplex/amplifying means for selectively demultiplexing apredetermined wavelength from input light and optically amplifying it isconnected to the input of the selective-demultiplex/amplifying means toform an optical loop. Thus, a single wavelength selected by theselective-demultiplex/amplifying means can be supplied as output light.That is, any one of a plurality of discrete wavelengths can be selected.Since it is selected from predetermined wavelengths, output with astable wavelength can be obtained. Since the polarization adjustingmeans in an appropriate location selects and maintains an appropriateplane of polarization for each element, behaviors of individual elementsare stabilized. If essential means are of a polarization holding type,polarization adjusting means may be omitted to reduce elements.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram showing a general construction of afirst embodiment of the invention;

FIG. 2 shows waveform characteristics of the same embodiment;

FIG. 3 is a schematic block diagram of a general construction of amodified embodiment;

FIG. 4 shows waveform diagrams of a version using AWGs as a wavelengthdemultiplexing element 10 and a wavelength multiplexing element 12, andcontaining two FSRs of these AWGs in the band of amplification of theoptical amplifier 14;

FIG. 5 shows a waveform obtained by an experiment;

FIG. 6 is a schematic block diagram showing a general construction of aversion commonly using two optical amplifiers 14-1 and 14-2;

FIG. 7 is a schematic block diagram showing a general construction of anembodiment configured to modulate multi-wavelength light collectively;

FIG. 8 is a schematic block diagram showing a general construction of anembodiment of the invention applied to a multi-wavelength ASE lightsource;

FIG. 9 is a schematic block diagram showing a general construction of anembodiment configured to modulate each wavelength componentindividually;

FIG. 10 is a schematic block diagram showing a general construction ofan embodiment configured to extract one or more desired wavelengthsamong a plurality of wavelengths in given wavelength intervals;

FIG. 11 is a schematic block diagram showing a general construction ofan embodiment applied to a wavelength-variable light source foroutputting a single discrete wavelength;

FIG. 12 shows distribution of wavelengths in output of the embodimentshown in FIG. 11;

FIG. 13 is a schematic block diagram showing a general construction ofan embodiment applied to a multi-wavelength mode lock pulse lightsource;

FIG. 14 is a schematic block diagram showing a general construction of awavelength converting apparatus using the wavelength-variable lightsource shown in FIGS. 10 and 12 as its pump light source;

FIG. 15 is a schematic block diagram showing a general construction of aconventional multi-wavelength light source; and

FIG. 16 is a characteristics diagram of the conventional light sourceshown in FIG. 15.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the invention are explained in detail with reference tothe drawings.

FIG. 1 is a schematic block diagram showing a general construction of afirst embodiment of the invention. FIG. 2 shows wavelengthcharacteristics of this embodiment.

In FIG. 1, reference numeral 10 denotes a wavelength demultiplexingelement for demultiplexing input light through an input port #1 into aplurality of predetermined wavelength components (in this embodiment,components of wavelengths λ1 to λ32). Numeral 12 denotes a wavelengthmultiplexing element for wavelength-multiplexing the light componentswith multiple wavelengths (in this embodiment, wavelengths λ1 to λ32).Namely, these elements are waveguide-type wavelength selecting filters(AWG). Other than AWG, known as another optical element fordemultiplexing and multiplexing a plurality of wavelengths collectivelyis the optical demultiplex/multiplexing filter developed by OpticalCorporation of America, U.S.A. Also this type of optical element can beused as the wavelength demultiplexing element 10 and the wavelengthmultiplexing element 12.

Output ports #1 through #32 of the wavelength demultiplexing element 10are connected to input ports #1 through #32 of the wavelengthmultiplexing element 12 via optical amplifiers 14 (14-1 through 14-32).Output port #1 of the wavelength multiplexing element 12 is connected toa fiber coupler 16, and one of two outputs of the fiber coupler 16 isconnected to input port #1 of the wavelength demultiplexing element 10.Thus, the other output of the fiber coupler 16 is extracted as desiredmulti-wavelength light. The unused output end of the fiber coupler 16 ismade as a non-reflective end. As a result, instable oscillation byFresnel reflection can be prevented. The same also applies to allembodiments shown below.

Each optical amplifier 14 includes an erbium-doped optical fiberamplifier, pumping light source and wavelength demultiplex/multiplexing(WDM) coupler for supplying output light from the pumping light sourceto the optical fiber amplifier. The optical amplifier 14 may be made ofa semiconductor laser amplifier and a Raman amplifier.

Briefly explained below are functions of AWG used as the wavelengthdemultiplexing element 10 and the wavelength multiplexing element 12.AWG is an optical element in which wavelengths λ1 through λ32 enteringin the input port #1 are output from output ports #1 through #32, andwavelengths λ1 through λ32 entering in the input port #2 are output fromoutput ports #2 through #32 and #1. Those entering in the subsequentports are output from output ports with corresponding and subsequentnumbers, and wavelengths λ1 through λ32 entering in the input port #32are output from output ports #32 and #1 to #31. Wavelength intervals ofwavelengths λ1 through λ32 are determined by the inner interferencestructure. Therefore, when wavelength-division multiplexed lightcontaining wavelengths λ1 through λ32 enters in the input ports #1,these wavelengths λ1 through λ32 are wavelength-demultiplexed and outputfrom corresponding output ports #1 through #32. In contrast, when lightsof wavelengths λ1 through λ32 enter in respective corresponding inputports #1 through #32, wavelength-multiplexed light containing theentered wavelengths λ1 through λ32 is output from the output port #1.

AWGs have a periodicity, and these wavelengths λ1 through λ32 areso-called basic waves. Also longer wavelengths λ1′ through λ32′ andshorter wavelengths λ31″′ and λ32″′ are wavelength-demultiplexed andwavelength-multiplexed.

FIG. 2(1) shows composite transparent wavelength characteristicsobtained when output ports of the wavelength demultiplexing element 10are connected to common-numbered input ports of the wavelengthmultiplexing element 12, respectively. In this embodiment, in which thewavelength demultiplexing element 10 and the wavelength multiplexingelement 12 are 32×32 type AWGs, the transparent wavelengthcharacteristics are periodic, and 32 wavelengths form one cycle, asexplained above. In general, this is defined as FSR (Free SpectralRange) of AWG. There are wavelengths λ1′, λ2′, . . . and . . . , λ31″′and λ32″′ outside λ1 through λ32 used in the embodiment, as shown inFIG. 2(1). For example, λ1′ and λ32″′ entering in the input port #1 ofthe wavelength demultiplexing element 10 are output from output ports #1and #32.

If the composite transparent wavelength characteristics of thewavelength demultiplexing element 10 and the wavelength multiplexingelement 12 are such that the transparent wavelength width for eachwavelength is sufficiently narrow, longitudinal modes of ring resonance,described later, can be decreased to a few or only one. This is attainedby narrowing the transparent wavelength width of each wavelength in thetransparent wavelength characteristics of the wavelength demultiplexingelement 10 and the wavelength multiplexing element 12, respectively, orby slightly shifting the transparent wavelength characteristics of thewavelength demultiplexing element 10 from those of the wavelengthmultiplexing element 12. The latter is advantageous for obtaining adesired wavelength width more easily although the loss is larger.

Ideally, each of the optical amplifiers 14-1 through 14-32 hasamplifying wavelength characteristics covering one cycle of FSR, namely,the wavelength range of λ1 through λ32, and preferably exhibiting adrastic decrease in gain beyond the range. Actually, in accordance withthe amplifying wavelength characteristics of available opticalamplifiers, AWGs having FSRs consistent with the amplifying wavelengthcharacteristics are used as the wavelength demultiplexing element 10 andthe wavelength multiplexing element 12. The gain of amplification byeach optical amplifier 14 is determined larger by approximately 10 dBthan the loss in one circulation of the loop made of the wavelengthdemultiplexing g element 10, optical amplifier 14, wavelengthmultiplexing element 12 and fiber coupler 16.

Basically, it is sufficient for each of the optical amplifiers 14-1through 14-32 that its gain center wavelength can cover a singlewavelength assigned to it. However, the use of different opticalamplifiers with different gain center wavelengths makes the process ofproducing and assembling respective element more troublesome, and it ispreferable to use optical amplifiers 14-1 to 14-32 with the sameamplifying wavelength range. From this point of view, the amplifyingwavelength characteristics in which the gain is flat throughout onecycle, namely one FSR OF AWGs 10 AND 12, and drastically decreasesoutside this range is preferable.

Due to the composite transparent wavelength characteristics of thewavelength demultiplexing element 10 and the wavelength multiplexingelement 12 (FIG. 2(1)) and the amplifying wavelength characteristics ofthe optical amplifiers 14-1 through 14-32 (FIG. 2(2)), the loop gain ofthe embodiment shown in FIG. 1 draws peaks at wavelengths λ1, . . . λ32,and light output from the fiber coupler 16 to the exterior of theoptical loop results in the spectrum shown in FIG. 2(3). Since thewavelength demultiplexing element 10 and the wavelength multiplexingelement 12 have the same transparent center wavelength and uniformtransmissivity to respective wavelengths, and the optical amplifiers14-1 through 14-32 have substantially the same gain, respectivewavelengths λ1 to λ32 in output light extracted by the fiber coupler 16have substantially the same optical intensity. In AWG, variance in lossamong different wavelengths upon wavelength division and wavelengthmultiplexing can be readily reduced to 3 through 4 dB or less in theprocess of fabrication, and this degree of variance can be compensatedby fine adjustment of amplification gains of respective opticalamplifiers 14-1 thorough 14-32.

Since each optical amplifier 14-1 to 14-32 amplifies a single wavelengthalone, mode competition does not occur, and stable amplification ofinput light is promised. Therefore, this embodiment can realizemulti-wavelength oscillation in wavelengths λ1 to λ32, and cansubstantially equalize intensities of respective wavelengths.

It is no problem, provided that one of the wavelength demultiplexingelement 10 and the wavelength multiplexing element 12, preferably thewavelength demultiplexing element 10, does not have a wavelengthperiodicity. However, under the conditions where the wavelengthdemultiplexing element 10 (and the wavelength multiplexing element 12)has a wavelength periodicity, and FSR of the wavelength demultiplexingcharacteristics is narrower than the amplification band of the opticalamplifiers 14, which results in containing two FSRs in the amplificationband of the optical amplifiers 14, the loop gain happens to exist alsofor wavelengths outside λ1 to λ32, e.g. wavelengths λ1′ and λ32″′, andpossibly causes mode competition or instable oscillation.

This can be prevented by locating an optical band pass filter in theloop to pass wavelengths λ1 to λ32 alone. FIG. 3 is a schematic blockdiagram showing a general construction of another embodiment taken forthis purpose. Reference numeral 20 denotes a wavelengthdemultiplex/amplify/multiplexing unit containing the wavelengthdemultiplexing element 10, wavelength multiplexing element 12 andoptical amplifiers 14 of FIG. 1. Numeral 22 denotes an optical band passfilter (optical BPF) transparent to wavelengths λ1 through λ32 alone inoutput light from the wavelength demultiplex/amplify/multiplexing unit20. Numeral 24 denotes a fiber coupler which demultiplexes output lightof the optical BPF 22 into two components, and supplies one to thewavelength demultiplex/amplify/multiplexing unit 20 and extracts theother as multi-wavelength output.

FIG. 4 shows waveforms of a version using AWGs as the wavelengthdemultiplexing element 10 and the wavelength multiplexing element 12,and containing two FSRs of AWGs in the amplification band of the opticalamplifiers 14. FIG. 4(1) shows transparent wavelength characteristics ofAWGs used as the wavelength demultiplexing element 10 and the wavelengthmultiplexing element 12. FIG. 4(2) shows amplification characteristicsof the optical amplifiers 14. FIG. 4(3) shows wavelength characteristicsof light passing through the optical amplifier 14-1 when the optical BPF22 is not provided. As shown, since three wavelengths λ1, λ1′ and λ1″pass through the optical amplifier 14-1 and are amplified, competitionof these wavelengths invites instable oscillation of the targetwavelength λ1.

FIG. 4(4) shows transparent characteristics of the optical BPF 22. FIG.4(5) shows wavelength characteristics of light passing through theoptical amplifier 14-1 when the optical BPF 22 is provided. Since theoptical BPF 22 permits the basic waves (λ1 to λ32) alone to circulate inthe loop, wavelength λ1 alone, here, can enter the optical amplifier14-1 and is amplified there.

In this manner, in the embodiment shown in FIG. 3, wavelengths otherthan basic waves (wavelengths λ1 through λ32) are removed by the opticalBPF 22, and do not circulate in the loop. Therefore, even if thewavelength demultiplexing characteristics of the wavelengthdemultiplexing element 10 (and the wavelength multiplexing element 12)are periodic such that it demultiplexes (or they demultiplex)wavelengths other than basic waves as well, and the optical amplifiers14 can amplify these undesired waves sufficiently, stablemulti-wavelength laser oscillation containing basic waves alone isensured.

Wavelength intervals of wavelengths contained in output light extractedfrom the fiber couplers 16 and 24 are determined by wavelengthselectivities of the wavelength demultiplexing element 10 and thewavelength multiplexing element 12. It is easy to design AWGs such thatthe wavelength intervals be 100 GHz (0.8 nm) or its integer multiple.Therefore, it is sufficiently possible to realize multi-wavelengthoscillation with wavelength intervals of approximately 0.8 nm.

FIG. 5 shows waveforms confirmed by an actual experiment. Used in theexperiment are AWGs of wavelength intervals of 0.7 nm as the wavelengthdemultiplexing element 10 and the wavelength multiplexing element 12.Four ports in every other sequences are connected to counterpart portshaving common numbers via optical amplifiers. It is known that fourwavelengths in intervals of 1.4 nm are oscillated simultaneously insubstantially the same optical intensity. The side mode suppressionratio was as good as 35 dB, and the ratio of the signal level to thebackground noise level was as good as approximately 60 dB.

A quartz AWG has a temperature coefficient of approximately 0.01 nm/° C.which is smaller by one digit than that of a semiconductor laser.Therefore, the accuracy for temperature control of two AWGs used as thewavelength demultiplexing element 10 and the wavelength multiplexingelement 12 can be alleviated to {fraction (1/10)} of the accuracyrequired for a signal-generating semiconductor laser for generatingsignal light used in wavelength multiplexing. Considering that thetemperature controlling accuracy of the pumping light source of theoptical amplifiers 14 (for example, a semiconductor laser for awavelength around 1.48 nm) need not be so high as that required for asignal-generating semiconductor laser, temperature control of thepumping light source can be simplified. That is, this embodiment makesthe entire temperature control easier and simpler, and can bemanufactured economically.

This embodiment also makes it easy to adjust and modify wavelengths inoutput light because, by selecting appropriate temperatures of AWGs usedas the wavelength demultiplexing element 10 and the wavelengthmultiplexing element 12, λ1 to λ32 can be shifted to longer or shorterwavelengths while maintaining the same wavelength intervals.

In most cases, the optical amplifiers 14 have their own pumping lightsources. However, erbium-doped optical fibers of a plurality of opticalamplifiers can be pumped by a single pumping light source. FIG. 6 is aschematic block diagram showing a general construction of the modifiedpart of a modified embodiment in this respect. Parts or elements commonto those of FIG. 1 are labelled with common reference numerals. Outputport #1 of the wavelength demultiplexing element 10 is connected toinput port #1 of the wavelength multiplexing element 12 via an opticalisolator 30-1, erbium doped optical fiber 32-1 and wavelengthdemultiplex/multiplexing (WDM) coupler 34-1. Similarly, output port #2of the wavelength demultiplexing element 10 is connected to input port#2 of the wavelength multiplexing element 12 via an optical isolator30-2, erbium-doped optical fiber 32-2 and wavelengthdemultiplex/multiplexing coupler 34-2.

Output light of a 1.48 μm pumping semiconductor laser 36 is divided intotwo parts by a 3 dB coupler 38, and one of them is supplied to theerbium-doped optical fiber 32-1 via the WDM coupler 34-1 while the otheris supplied to the erbium-doped optical fiber 32-2 via the WDM coupler34-2. The optical isolators 30-1, 30-2 prevent that pumping light to theerbium-doped optical fibers 32-1, 32-2 enter the output ports #1 and #2of the wavelength demultiplexing element 10.

In this manner, the optical amplifiers 14-1 and 14-2 can share a singlepumping light source. By using this arrangement also for other opticalamplifiers 14-3 through 14-32, the total number of pumping light sourcescan be reduced to a half.

It is convenient to use light containing collectively modified multiplewavelengths in transmission tests of wavelength division multiplexoptical transmission systems. Explained below is an embodiment in whichmulti-wavelength light is modified collectively. FIG. 7 is a schematicblock diagram of its general construction. Numeral 40 denotes awavelength demultiplex/amplify/multiplexing unit containing thewavelength demultiplexing element 10, optical amplifiers 14 andwavelength multiplexing element 12 of FIG. 1 (and optical BPF 22 of FIG.3). Output light of the unit 40 enters in a fiber coupler 44 via apolarizer 42. One of outputs of the fiber coupler 44 is fed to thewavelength demultiplex/amplify/multiplexing unit 40 through apolarization adjuster 46 and a depolarizer 48. The other output of thefiber coupler 44 enters into the external optical modulator 52 through apolarization adjuster 50.

In a fiber ring or loop formed by the wavelengthdemultiplex/amplify/multiplexing unit 40, polarizer 42, fiber coupler44, polarization adjuster 46 and depolarizer 48, laser oscillation ofmultiple wavelengths occur simultaneously in substantially the sameintensity in the same manner as the embodiment shown in FIG. 1. Themulti-wavelength light is extracted from the fiber ring by the fibercoupler 50.

Also in the wavelength demultiplex/amplify/multiplexing unit 40, thecomposite transparent wavelength characteristics of the wavelengthdemultiplexing and the wavelength multiplexing are chosen tosufficiently narrow the transparent wavelength widths for individualwavelengths so that longitudinal modes can be decreased to a few or onlyone. As explained with reference to FIG. 1, this is attained bynarrowing the transparent wavelength width of each wavelength in thetransparent wavelength characteristics of each of the wavelengthdemultiplexing element and the wavelength multiplexing element, or byslightly shifting the transparent wavelength characteristics of thewavelength demultiplexing element from those of the wavelengthmultiplexing element.

The use of the polarizer 42 contributes to suppression of polarizationfluctuations in the fiber ring. In order to prevent interference in thewavelength demultiplex/amplify/multiplexing unit 40, the depolarizer 48depolarizes the input light. If the polarized condition by the polarizer42 is maintained, interference or other undesirable effects may occur inthe external modulator 52. To remove such trouble in the externalmodulator 52, the polarization adjuster 50 adjusts the polarization.Additionally, for more effective depolarization by the depolarizer 48,the polarization adjuster 46 adjusts polarization of the input light.

While light circulates in the fiber ring made of the wavelengthdemultiplex/amplify/multiplexing unit 40, polarizer 42, fiber coupler44, polarization adjuster 46 and depolarizer 48, simultaneous laseroscillation in multiple wavelengths occurs in the same manner as theembodiment of FIG. 1. The multi-wavelength light by the simultaneouslaser oscillation is extracted by the fiber coupler 44, and applied tothe external modulator 52 via the polarization adjuster 50. The externaloptical modulator 52 modulates the applied multi-wavelength lightcollectively in accordance with an externally applied modulation signal.The modulated light is supplied to transmission optical fibers, etc.

Polarization fluctuation in the fiber ring causes fluctuation of thering cavity mode, and makes simultaneous oscillation of multiplewavelengths instable. In the embodiment, however, since the polarizer 42suppresses fluctuations in planes of polarization, instable oscillationcan be suppressed. If, however, the wavelengthdemultiplex/amplify/multiplexing unit 40 (wavelength demultiplexingelement 10, wavelength multiplexing element 12 and optical amplifiers14) and the fiber coupler 16 are of a polarization holding type, thepolarizer 42, depolarizer 48 and polarization adjusters 46, 50 may beomitted.

Also in the embodiment shown in FIG. 7, if looping of light of undesiredwavelengths outside the target wavelength band should be previouslyprevented, an optical BPF similar to the optical BPF 22 in theembodiment shown in FIG. 3 is placed at a desired location in the fiberring (inside or outside the wavelength demultiplex/amplify/multiplexingunit 20).

To test characteristics of optical components, it is desirable to use anASE (Amplified Spontaneous Emission) light source for multiplewavelengths, which does not laser-oscillate. Such a multi-wavelength ASElight source can be readily obtained according to the invention. FIG. 8is a schematic block diagram showing a general construction of anembodiment taken for this purpose.

Explained below is the construction of the embodiment of FIG. 8. Numeral60 denotes a wavelength demultiplex/amplify/multiplexing unit similar tothe wavelength demultiplex/amplify/multiplexing unit 40. Output lightfrom the unit 60 enters in an acousto-optic modulator 64 via apolarization adjuster 62. Output of an A/O modulator 64 enters in afiber coupler 70 via a polarization adjuster 66 and a depolarizer 68.One of outputs of the fiber coupler 70 enters in the wavelengthdemultiplex/amplify/multiplexing unit 60, and the other output of thefiber coupler 70 is extracted as multi-wavelength ASE light.

The A/O modulator 64 slightly shifts and outputs wavelengths in theinput light. Therefore, light circulating in the fiber ring made of thewavelength demultiplex/amplify/multiplexing unit 60, polarizationadjuster 62, A/O modulator 64, polarization adjuster 66, depolarizer 68and fiber coupler 70 is slightly shifted in wavelength by the A/Omodulator 64. As a result, laser oscillation does not occur, andamplified spontaneous emission light, that is, ASE light is obtained.Since the multi-wavelength state is not lost even after passing the A/Omodulator 64, the light extracted from the fiber coupler 70 is ASE lightcontaining multiple wavelengths.

In order to prevent interference or other undesired events in the A/Omodulator 64, the polarization adjuster adjusts polarization of inputlight to the A/O modulator 64. If the output light of the A/O modulator64 remains in a specific polarized state, undesirable effects may occurin the wavelength demultiplex/amplify/multiplexing unit 60. To deal withthe matter, the polarization adjuster 66 and depolarizer 68 previouslycancel the specific polarized state. The polarization adjuster 66 andthe depolarizer 68 may be located between the fiber coupler 70 and thewavelength demultiplex/amplify-multiplexing unit 60. However, as shownin FIG. 8, when they are located between the A/O modulator 64 and thefiber coupler 70, polarization dependency is removed frommulti-wavelength ASE light extracted from the fiber coupler 70, and thislight can be used more conveniently for examining variouscharacteristics (such as amplification characteristics or losscharacteristics) of optical components to wavelength-divisionmultiplexed light.

In the embodiment shown in FIG. 7, multi-wavelength light is modifiedcollectively. However, it is preferable that individual wavelengths canbe data-modulated independently for use in actual transmission tests ortransmission.

FIG. 9 is a schematic block diagram showing a general construction of anembodiment configured to modify respective wavelengths individually.Numeral 80 denotes a wavelength demultiplexing element similar to thewavelength demultiplexing element 10, and 82 denotes a wavelengthmultiplexing element similar to the wavelength multiplexing element 12.Output ports of the wavelength demultiplexing element 80 are connectedto common-numbered input ports of the wavelength multiplexing element 82through optical amplifiers 84 (84-1 through 84-32) similar to theoptical amplifiers 14. wavelength multiplex output of the wavelengthmultiplexing element 82 is connected to the input of the wavelengthdemultiplexing element 80. Here again, if necessary, an optical BPFsimilar to the optical BPF 22 used in the embodiment of FIG. 3 may beprovided, for example, between the output of the wavelength multiplexingelement 82 and the input of the wavelength demultiplexing element 80.

Since this embodiment does not take out multi-wavelength light directly,it does not use a fiber coupler similar to the fiber coupler 16.Instead, fiber couplers 86 (86-1 through 86-32) are provided fordividing outputs of the optical amplifiers 84-1 through 84-32. Opticaloutputs extracted by the fiber couplers 86-1 to 86-32 are applied toexternal modulators 88 (88-1 through 88-32). The external modulators 88(88-1 through 88-32) are supplied with different modulation signals #1through #32. Optical outputs from the external modulators 88-1 through88-32 are applied to a wavelength multiplexing element 90 which isidentical to the wavelength multiplexing element 82.

The wavelength demultiplexing element 80, wavelength multiplexingelement 82, optical amplifiers 84 and fiber coupler 86 are of apolarization holding type. If not, additional elements corresponding tothe polarizer 42, polarization adjuster 46 and depolarizer 48 used inthe embodiment of FIG. 7 must be provided in the loop made of thewavelength demultiplexing element 80, optical amplifiers 84 andwavelength multiplexing element 82.

Composite transparent wavelength characteristics of the wavelengthdemultiplexing element 80 and the wavelength multiplexing element 82 arechosen to sufficiently narrow the transparent wavelength widths forindividual wavelengths so that longitudinal modes can be decreased to afew or only one. As explained with reference to FIG. 1, this is attainedby narrowing the transparent wavelength width for each wavelength of thetransparent wavelength characteristics of each of the wavelengthdemultiplexing element 80 and the wavelength multiplexing element 82, orby slightly shifting the transparent wavelength characteristics of thewavelength demultiplexing element 80 from those of the wavelengthmultiplexing element 82.

Explained below are behaviors of the embodiment shown in FIG. 9. In theloop made of the wavelength demultiplexing element 80, opticalamplifiers 84 and wavelength multiplexing element 82, laser oscillationof multiple wavelengths occur simultaneously in substantially the sameintensity in the same manner as the embodiment shown in FIG. 1.Respective wavelengths by laser oscillation are taken out individuallyby the fiber couplers 86-1 through 86-3, and applied to the externalmodulators 88-1 through 88-32. External modulators 88-1 and 88-32modulate their optical inputs by modulation signals #1 to #32,respectively. As a result, modulated optical outputs containingdifferent wavelengths modulated by different modulation signals #1 to#32 can be obtained. Then, the wavelength multiplexing element 90wavelength-multiplexes the outputs of the external modulators 88-1through 88-32, and supplies the multiplexed light to an external elementsuch as optical fiber transmission path, for example. Thus, thetransmission test can be executed in practical conditions fortransmission.

It is essential for the wavelength multiplexing element 90 only tocompose or multiplex optical outputs of the external modulators 88-1through 88-32, and it need not have the same wavelength multiplexingfunction as that of the wavelength multiplexing element 82.

In some applications, it is desired to take out one or some wavelengthsfrom a number of wavelengths in certain wavelength intervals. Suchrequirement is attained by modifying the embodiment of FIG. 1 in themanner as shown in FIG. 10. That is, optical switches 92 (92-1 through92-32) are inserted between outputs of optical amplifiers 14-1 through14-32 and input ports of the wavelength multiplexing element 12. Whenone or more of the optical switches 92-1 through 92-32 are turned on,corresponding wavelengths alone circulate in the fiber ring, and laseroscillated outputs with corresponding wavelengths are taken out from thefiber coupler 16. For example, when only the optical switch 92-4 isturned on, only the wavelength λ4 stimulates laser oscillation, and thelaser light is taken out from the fiber coupler 16. If optical switchesin every two intervals are turned on among optical switches 92-1 through92-32, then multi-wavelength light containing wavelengths in wavelengthinterval twice that of the wavelength demultiplexing element 10 (and thewavelength multiplexing element 12) can be obtained.

In the same manner as the embodiment shown in FIG. 3, which is amodified version of the embodiment of FIG. 1, an optical BPF similar tothe optical BPF 22 in the embodiment of FIG. 3 may be provided, ifnecessary.

According to the embodiment shown in FIG. 10, light containing only oneor some of a plurality of predetermined wavelengths can be obtained.That is, this light source can be operated as adiscrete-wavelength-variable light source or as a multi-wavelength lightsource capable of selecting any desired wavelength interval.

The modification in the embodiment shown in FIG. 10 is applicable alsoto embodiments shown in FIGS. 7, 8 and 9.

FIG. 11 is a schematic block diagram showing a general construction ofan embodiment realizing a wavelength-variable light source for adiscrete single wavelength. Numeral 110 denotes a wavelengthdemultiplexing element similar to the wavelength demultiplexing element10, and 112 denotes a wavelength multiplexing element similar to thewavelength multiplexing element 12. 114 designates a 32×1 optical switchfor selecting one of plural output ports (32 output ports in thisembodiment) of the wavelength demultiplexing element 110. 116 denotes anoptical amplifier for amplifying output light from the optical switch114. 118 denotes a 1×32 optical switch for switching an output of theoptical amplifier 116 to one of plural input ports (32 input ports inthis embodiment) of the wavelength multiplexing element 112.

Optical switches 114, 118 can be turned ON and OFF by using a commonswitching signal. That is, optical switches 114, 118 select an outputport and an input port with a common number among plural output ports ofthe wavelength demultiplexing element 110 and plural input ports of thewavelength multiplexing element 112.

Since the optical amplifier 116 amplifies one of wavelengths λ1 throughλ32 demultiplexed by the wavelength demultiplexing element 110, itsamplification band is wide enough to cover wavelengths λ1 through λ32and need not be wider. No problem of FSR occurs.

Explained below are behaviors of the embodiment shown in FIG. 11. Amongwavelengths λ1 through λ32 demultiplexed by the wavelengthdemultiplexing element 110, a wavelength selected by the optical switch114 is amplified by the optical amplifier 116. Output of the opticalamplifier 116 enters in one of input ports of the wavelengthmultiplexing element 112, having a number common to the output portselected by the optical switch 114. Therefore, the wavelengthmultiplexing element 112 outputs light amplified by the opticalamplifier 116 from its output port to the fiber coupler 120. The fibercoupler 120 divides the light from the wavelength multiplexing element112 into two components, and supplies one to the wavelengthdemultiplexing element 110 and externally outputs the other as outputlight.

The light of the wavelength selected by the optical switches 114, 118circulates in the fiber ring made of the wavelength demultiplexingelement 110, optical switch 114, optical amplifier 116, optical switch118, wavelength multiplexing element 112 and fiber coupler 120, andstimulates laser oscillation.

FIG. 12 shows an example of wavelength distribution in output of theembodiment shown in FIG. 11. In this example, an output port #i of thewavelength demultiplexing element 110 and an input port #i of thewavelength multiplexing element 112, which correspond to wavelength λ1,are selected by the optical switches 114, 118. In FIG. 12, the actuallylaser-oscillated wavelength is shown by the bold solid line, andwavelengths that can be selected are shown by the thin solid line.

If a sufficient wavelength selectivity is ensured only with thewavelength demultiplexing element 110, the system may omit thewavelength multiplexing element 112 and hence the optical switch 118.

FIG. 13 is a schematic block diagram showing a general construction of amulti-wavelength mode-locked pulse light source taken as anotherembodiment of the invention. In a pulse light source, it is desirablethat the pulse phase is stable on the time domain. In this embodiment,mode-locked pulse light for plural wavelengths can be obtainedcollectively.

Numeral 130 denotes a wavelength demultiplex/amplify/multiplexing unitcomprising the wavelength demultiplexing element 10, optical amplifiers14 and wavelength multiplexing element 12, all of FIG. 1, wavelengthdemultiplexing element 10, optical amplifiers 14-1 through 14-32,optical switches 92-1 through 92-32 and wavelength multiplexing element12, all of FIG. 10, or wavelength demultiplexing element 110, opticalswitch 114, optical amplifier 116, optical switch 118 and wavelengthmultiplexing element 112, all of FIG. 11. When the wavelengthdemultiplex/amplify/multiplexing unit 120 comprises the wavelengthdemultiplexing element 10, optical amplifiers 14 and wavelengthmultiplexing element 12 of FIG. 1, laser oscillation occurssimultaneously in multiple wavelengths. When the unit 120 comprises thewavelength demultiplexing element 10, optical amplifiers 14-1 through14-32, optical switches 92-1 through 92-32 and wavelength multiplexingelement 12 of FIG. 10, or the wavelength demultiplexing element 110,optical switch 114, optical amplifier 116, optical switch 118 andwavelength multiplexing element 112, laser oscillation occurs inselected one or some wavelengths.

Numeral 134 denotes an electroabsorption optical modulator formodulating output light of the wavelengthdemultiplex/amplify/multiplexing unit by a sinusoidal modulation signal,and 136 denotes a fiber coupler for dividing output light of theelectroabsorption optical modulator 134 into two parts to supply one tothe wavelength demultiplex/amplify/multiplexing unit 130 and toexternally output the other as output light.

When L is the ring length of the ring or loop made of the wavelengthdemultiplex/amplify/multiplexing unit 130, electroabsorption opticalmodulator 134, and fiber coupler 136, n is the effective refractiveindex, and c is the light velocity, a sinusoidal voltage of a frequencycorresponding to an integer multiple of the basic frequency of=c/(nL) isapplied as a modulation signal to the electroabsorption opticalmodulator 134. Transparent band widths of the wavelength demultiplexingelement (and wavelength multiplexing element) in the wavelengthdemultiplex/amplify/multiplexing unit 130 for respective wavelengths aredetermined to be sufficiently narrower than the circulating basicfrequency of.

Under these conditions of frequency, light circulating in the fiber ringor loop made of the wavelength demultiplex/amplify/multiplexing unit130, electroabsorption optical modulator 134 and fiber coupler 136 ismode-locked to the sinusoidal modification signal applied to theelectroabsorption optical modulator 134, and has the form of a pulserising at an apex or nadir of the sinusoidal modulation signal. As aresult, a sequence of pulses containing multiple wavelengths andmode-locked can be obtained.

Since the ring length L and the effective refractive index n vary fordifferent wavelengths, it is necessary, in a strict sense, to adjusteffective optical path lengths for individual wavelengths in thewavelength demultiplex/amplify/multiplexing unit 130. However, it issufficient to connect an electroabsorption optical modulator (and, ifnecessarily, a polarization adjuster) in a location anterior to theoptical path for each wavelength, more preferably, anterior to theoptical amplifier 14, in the wavelength demultiplex/amplify/multiplexingunit 130 and to apply a sinusoidal modulation signal in correspondingphase and frequency to the electroabsorption optical modulator tomodulate it there. Then, the phase and frequency of one sinusoidalsignal may be adjusted independently, and may be applied as a modulationsignal to each electroabsorption optical modulator. In this case,however, A number of electroabsorption optical modulators (andpolarization adjusters) corresponding to respective wavelengths areneeded and the light source becomes more expensive than the embodimentof FIG. 13.

In the embodiment of FIG. 13, when looping of undesired wavelengthsother than the target wavelengths should be prevented, an optical BPFsimilar to the optical BPF 22 in the embodiment shown in FIG. 3 isprovided at an appropriate location, for example, between the output ofthe wavelength demultiplex/amplify/multiplexing unit 130 and the opticalmodulator 134.

By using the wavelength variable light source according to theembodiment shown in FIG. 10 or FIG. 11 as a pump light source of awavelength converting apparatus, any wavelength acceptable in a networkcan be used efficiently. FIG. 14 is a schematic block diagram of ageneral construction of an embodiment taken for this purpose.

In FIG. 14, numeral 140 denotes a wavelength variable light source shownin FIG. 10 or FIG. 11, which is designed and fabricated so thatwavelengths acceptable in a wavelength-division multiplexing opticalnetwork can be selected. Output light from the wavelength variable lightsource is applied as pumping light λp to a semiconductor laser amplifier142. On the other hand, input modified light λs enters into a terminal Aof an optical circulator 144. The optical circulator 144 is an opticalelement which outputs the light entering through the terminal A fromanother terminal B and outputs the light entering through the terminal Bfrom a terminal C. Output light from the terminal B of the opticalcirculator 144 (modulated light λs) is fed to an end surface of thesemiconductor laser amplifier 142 opposite from the end surface intowhich the pumping light λp is entered.

The pumping light λp and the modulated light λs travel in oppositedirection within the semiconductor laser amplifier 142. If the intensityof the pump light λp is held at a value where the gain of thesemiconductor laser amplifier 142 is saturated, the pumping light λp iswaveform-modified in accordance with the intensity waveform of themodulated light λs due to their mutual gain modulation effect. That is,waveform of the pumping light λp becomes substantially opposite from thewaveform of the modulated light λs. The waveform-modified pumping lightλp enters into the optical circulator 144 through the terminal B, and itis output from the terminal C. The light output from the terminal C ofthe optical circulator 144 has a form in which the input modulated lightλs has been wavelength-converted to the wavelength of the pump light λp.

In the discrete-wavelength-variable light source 140 to which theinvention is applied, its available wavelengths can be readily set tocoincide with wavelengths acceptable in the wavelength-divisionmultiplexing optical network. Once the wavelengths are set so, thewavelength of the optical signal obtained by wavelength conversion ofthe input modified signal λs is an acceptable wavelength of the network,and the acceptable wavelength in the network can be re-used. When alight source capable of varying continuous wavelengths, such asconventional multi-electrode semiconductor laser, for example, is usedas the wavelength variable light source 140, precisely accurate controlmust be made to adjust the wavelength of its output light to one ofwavelengths acceptable in the network, and this invites a muchcomplicated construction and a high cost. In contrast, according to theinvention, the discrete-wavelength-variable light source can select anappropriate wavelength through the switch, and can remove the need forwavelength control and severe accuracy therefor.

In addition to the foregoing examples, there are arrangements forfour-wave-mixing, for example, as wavelength converting mechanisms, anda fiber amplifier is also usable in lieu of the semiconductor laseramplifier and the arrangement using an absorption-type optical modulatoris disclosed in a patent application by the same Applicant, entitledWaveform Converting Apparatus, (Japanese Patent Application Heisei8-233796).

As readily understandable from the above explanation, according to theinvention, laser output containing multiple wavelengths withsubstantially uniform intensity can be obtained. By using as wavelengthdemultiplexing means an element for wavelength-demultiplexing inputlight into multiple wavelengths in predetermined wavelength intervals, amulti-wavelength light source for generating light containing multiplewavelengths in certain intervals can be realized. Since the light sourcehas a simple structure and is mostly of passive elements, it is stableagainst changes in temperature.

By intensity-modifying circulating light in an optical loop with amodification signal having an integer multiple frequency of the circularfrequency of the optical loop, multi-wavelength pulse light locked withthe modulation signal can be obtained.

By locating means posterior to wavelength division (preferably,posterior to optical amplification) for selectively supplying light toor blocking light from wavelength multiplexing means, multiplex outputlight containing any selected one or more wavelengths can be obtained.

According to the invention, it is also easy to modify multi-wavelengthlaser light either collectively or individually.

When wavelength shifting means is placed in an optical loop, an ASElight source for multiple wavelengths can be realized.

When the output of selectively demultiplexing and amplifying means forselectively demultiplexing a predetermined wavelength from input lightand amplifying the demultiplexed light is connected to input of the samemeans so as to form an optical loop, one of a plurality of discretewavelengths can be used as output light. That is, one of discretewavelengths can be selected. Since the wavelength is selected frompredetermined wavelengths, output containing stable wavelengths can beobtained.

What is claimed is:
 1. A discrete-wavelength-variable light source forselectively supplying at least one discrete wavelength in output laserlight, comprising: a selectively demultiplex/amplifier for selectivelydemultiplexing at least one predetermined wavelength from an input lightand optically amplifying them, wherein the selectivedemultiplex/amplifier comprises a wavelength demultiplexer todemultiplexer the input light into a plurality of predeterminedwavelengths, a first optical switch to select one of said wavelengthsfrom said wavelength demultiplexer, an optical amplifier to amplify anoptical output of the first optical switch, a wavelength multiplexer towavelength-multiples a plurality of optical inputs, and a second opticalswitch to supply an optical output of the optical amplifier to an inputof said wavelength multiplexer corresponding to the wavelength selectedby the first optical switch; and an optical splitter to split part of anoptical output from the selective demultiplex/amplifier to transfer itto the selective demultiplex/amplifier and to externally supply aremainder of the optical output from said selectivedemultiplex/amplifier.
 2. The discrete-wavelength-variable light sourceaccording to claim 1 wherein the wavelength demultiplexer demultiplexesthe input light into predetermined wavelengths in predeterminedwavelength intervals.
 3. The discrete-wavelength-variable light sourceaccording to claim 2 wherein the wavelength demultiplexer is awaveguide-type wavelength selecting filter.
 4. Thediscrete-wavelength-variable light source according to claim 1 whereinthe wavelength multiplexer is a wave-guide type wavelength selectingfiber.
 5. The discrete-wavelength-variable light source according toclaim 1 further comprising an optical modulator provided in an opticalloop, the loop comprising the selective multiplex/amplifier and saidoptical spitter, to intensity-modulate circulating light in said opticalloop in accordance with a modulation signal, the modulation signalhaving a frequency which is multiple integer of a frequency circulatingin the optical loop, the remainder of the optical output from the outputfrom the optical splitter being pulsating light.
 6. Thediscrete-wavelength-variable light source according to claim 5 whereinthe selective demultiplexer/amplifier and the optical splitter are apolarization holding type.
 7. The discrete-wavelength-variable lightsource according to claim 1 further comprising a polarization adjusterlocated adjacent to an input of the optical modulator.
 8. Thediscrete-wavelength-variable light source for selectively supplying atleast one discrete wavelength in output laser light, comprising: aselective demultiplex/amplifier for selectively demultiplexing at leastone predetermined wavelength from an input light and opticallyamplifying them; an optical splitter to split part of an optical outputfrom the selective demultiplex/amplifier to transfer it to the selectivedemultiplex/amplifier and to externally supply a remainder of theoptical output from said selective demultiplex/amplifier; an opticalmodulator provided in an optical loop, the loop comprising the selectivedemultiplex/amplifier and said optical splitter, to intensity-modulatecirculating light in said optical loop in accordance with a modulationsignal, the modulation signal having a frequency which is a multipleinteger of a frequency circulating in the optical loop, the remainder ofthe optical output from the optical splitter being pulsating light; anda polarization adjuster located adjacent to an input of the opticalmodulator.
 9. The discrete-wavelength-variable light source according toclaim 8 wherein the selective demultiplex/amplifier comprises awavelength demultiplexer to demultiplex the input light into a pluralityof predetermined wavelengths, a first optical switch to select one ofsaid wavelengths from said wavelength demultiplexer, an opticalamplifier to amplify an optical output of the first optical switch. 10.The discrete-wavelength-variable light source according to claim 9wherein the selective demultiplex/amplifier further comprises awavelength multiplexer to wavelength-multiplex a plurality of opticalinputs, and a second optical switch to supply an optical output of theoptical amplifier to an input of said wavelength multiplexercorresponding to the wavelength selected by the first optical switch.11. The discrete-wavelength-variable light source according to claim 10wherein the wavelength multiplexer is a wave-guide type wavelengthselecting filter.
 12. The discrete-wavelength-variable light sourceaccording to claim 9 wherein the selective demultiplex/amplifier theinput light input predetermined wavelengths in predetermined wavelengthintervals.
 13. The discrete-wavelength-variable light source accordingto claim 12 wherein the wavelength demultiplexer is a waveguide-typewavelength selecting filter.
 14. The discrete-wavelength-variable lightsource according to claim 8 wherein the selectivedemultiplexer/amplifier and the optical splitter are a polarizationholding type.